Because of its relative abundance in the air of the atmosphere, relatively large quantities of oxygen, as well as nitrogen and other gases, readily dissolve into various liquid media. The presence of dissolved oxygen, in particular, in some liquids, such as hydrocarbon fuels, may be objectionable because it supports oxidation reactions that yield undesirable by-products.
For instance, jet fuel in aircraft may be used as a coolant for various systems in the aircraft. When air-saturated fuel is heated to temperatures above about 120° C. (250° F.) to 150° C. (300° F.), the dissolved oxygen reacts to form free radical species (coke precursors) which initiate and propagate other autoxidation reactions leading to the formation of objectionable deposits, called “coke” or “coking”. As fuel temperature increases beyond about 150° C. (300° F.), the process of autoxidation consumes oxygen and forms carbonaceous deposits. The temperature at which autoxidation begins differs for different fuels. These autoxidation reactions may also occur in jet fuel as it is heated immediately prior to injection for combustion, such that deposits may occur in the injectors. In any event, the formation of such deposits impairs the normal functioning of a fuel delivery system, either with respect to an intended heat exchange function or the efficient injection of fuel.
A fuel stabilization unit reduces the amount of oxygen dissolved within a fuel. Reducing the amount of oxygen in a fuel increases the maximum allowable temperature of the fuel, thereby increasing its heat sink capacity when used for cooling components onboard the aircraft. One method of removing dissolved oxygen from fuels is by using a semi-permeable membrane deoxygenator. In a membrane deoxygenator, fuel is pumped over an oxygen permeable membrane. As the fuel passes over the membrane, a partial oxygen pressure differential across the membrane promotes the transport of oxygen out of the fuel through the membrane. Exemplary deoxygenators remove oxygen to a level at least below that at which significant coking would otherwise occur. As used herein, “significant coking” is the minimum amount of coking which, if it occurred in the interval between normal intended maintenance events for such portions of the fuel system, would be viewed as objectionable. Such coking occurs most readily in the portions of the fuel system having high temperatures and/or constricted flow paths.